The Fire Service says it is actively working to resolve faults identified in Fraser MAN engines, and is disputing claims of a failure at an Auckland house fire.
Firefighters in the city have blacklisted their entire fleet of the engines over safety concerns.
The Professional Firefighters' Union said a crew was at a house fire in Manurewa last week when an electrical fault caused a pump to lose water pressure.
Spokesman Boyd Raines said the lack of water could have had serious consequences, but fortunately the firefighters got out of the house safely.
He said the MAN units have had a myriad of problems and its members were banning the use of Auckland's fleet of 11 until they were proven safe.
Fire Service spokesman Larry Cocker said the union's account was not correct.
"Firefighters had been at an oven fire. They had finished and were packing up to go home, they were discussing whether some changes made to the engine had been effective, so they carried out a test and that is when they were able to identify a fault.
"Firefighters were not in a burning building when the fault occurred."
Mr Cocker said the engine that had the fault was being used to make changes and test fixes.
"As soon as the fault was identified, which was with the braking system and the cruise control, the engine was taken out of service until the fault was fixed."
The fault was fixed on units in Auckland, Hamilton and Christchurch yesterday and was being addressed in Palmerston North and Wellington today.
All fixes would be stress tested next month, Mr Cocker said.
"We're working with the union to ensure our equipment is safe - as with any new technology, it has thrown up some glitches, and as they come up we are fixing them.
"They cost a a lot of money and it is frustrating for us but we have to work through it, to make sure we get a good, safe and effective product."
There were 47 MAN units nationwide, he said. The engines were introduced in early 2015.
There would be no interruption to normal response levels as a result of the issue.
Firefighters have lost faith in engines, union says
Earlier, Mr Raines said the house fire had been the turning point for its Auckland members.
"The incident on Friday eroded the last vestiges of trust our members had in these vehicles."
Mr Raines said there were enough 20 to 30-year-old relief fire trucks in the city to fill the gaps. But it only needed one to break down, he said.
The Fire Service was recently forced to recall helmets, chemical splash suits and high pressure hoses because of faults and complaints from firefighters.
The union said a photo of the Fire Service's workshop in Ōtāhuhu showed six faulty trucks abandoned on the forecourt, with a further three, which were out of order, sitting in the yard.
Mr Raines said two remaining trucks, based at Grey Lynn and Remuera, were backed up by neighbouring stations running older, but more reliable, trucks.
"Residents living in Grey Lynn and Remuera are Fire Service levy-payers too and deserve better.
"The risk to firefighters and the public is too great," Mr Raines said.
Last year, for the first time in 20 years, firefighters banned a truck because of safety concerns. Since then, 11 trucks have been taken off the road in Auckland, while Christchurch and Wellington have each blacklisted one truck.